It is advantageous to provide various articles, for example, lenses, cathode ray tubes, optical displays, window films and windshields, with antireflection films which reduce the amount of light reflected from the surface of the article and thus reduce or eliminate “ghost” images formed by such reflected light. In addition, it is beneficial to provide various articles, such as optical displays, with protective films and/or antisoiling films in order to reduce the amount of contamination or damage to the surface of the article. For example, optical displays used in devices such as personal digital assistants (“PDAs”), cell phones, touch-sensitive screens, flat panel displays and removable computer filters are frequently subjected to handling and contact with a user's face, fingers, stylus, jewelry and/or other objects. Consequently, facial oils can adversely affect contrast, color saturation or brightness of an optical display. Scratches, abrasion, smudges and stains resulting from routine use can also cause the optical display to lose resolution and clarity, and sometimes to become unreadable or inoperative.
Antireflection coatings on a substrate typically comprise a plurality of inorganic layers, for example a metal or metal oxide layer and a silica layer. (The term “silica” is used herein in accordance with its normal meaning in the antireflection art to mean a material of the formula SiOx where x is not necessarily equal to two. As those skilled in the art are aware, such silica layers are often deposited by chemical vapor deposition, vacuum deposition, or sputtering of silicon in an oxygen atmosphere, so that the material deposited does not precisely conform to the stoichiometric formula SiO2 of pure silica.) Typically, one surface of a silica layer is exposed, and this exposed surface, which has a high surface energy as shown by its low contact angle with water, is highly susceptible to fingerprints and other marks. Such marks are extremely difficult to clean, often requiring the use of chemical cleaners.
An effective antireflection film is available commercially from Southwall Technologies (Palo Alto, Calif.). This material comprises a 180 μm poly(ethylene terephthalate) substrate provided with an abrasion-resistant hardcoat, and then successively with a 17 nm indium tin oxide (ITO) layer, a 23 nm silica layer, a 95 nm ITO layer, an 84 nm silica layer and finally a thin “lubrication” layer, which is formed from a fluoropolymer and is stated to improve the scratch resistance and to decrease the susceptibility of the surface to marking.
This complex film possesses excellent antireflection characteristics, but is so expensive (approximately US$10 per square foot, US$100 m−2) as to preclude its use in many applications where antireflection films are desirable. Much of the high cost of this film can be attributed to the 95 nm ITO layer and 84 nm silica layer, since these layers are typically formed by sputtering, and the cost of a sputtered layer is directly proportional to its thickness. Furthermore, if it is desired to produce large quantities of such a complex film on a production line basis, the need for four separate sputtering stations, all of which must be maintained under high vacuum, results in a complex and costly apparatus.
It has now been found that by providing a “thick” (i.e., optically active) polymer layer of carefully controlled refractive index above an inorganic layer or layers, the thickness(es) of the inorganic layer(s) can be greatly reduced, thereby reducing the overall cost of the antireflection coating, especially when the inorganic layer(s) is/are applied by a process such as sputtering or chemical vapor deposition in which the residence time of the substrate within the coating apparatus is directly proportional to the thickness of the required layer. Also, an antireflection coating using such a thick polymer layer, which can readily be applied with good uniformity by solution or other coating techniques, has good scratch or abrasion resistance and good stain, smudge, and soil resistance.